Significance Recombinant SHIVenv clones are a means to analyze the role of HIV-1 env genes in vivo for viral transmission across mucosal membranes and for SAIDS pathogenesis. Additionally, vaccines based on HIV-1 immunogens can be tested for efficacy by challenge with such recombinant viral clones. Objectives An animal model to study both HIV-1 infection and AIDS pathogenesis is not available. To analyze function of specific HIV-1 genes in vivo, SIV/HIV-1 recombinant viruses (designated SHIV) have been constructed by replacing genes in the pathogenic clone SIVmac239 with counterpart HIV-1 genes. We have made SHIV clones containing the envelope (env) gene of various HIV-1 subtype-B isolates, and have analyzed these recombinant viruses in vivo in juvenile and newborn macaques. Results A pathogenic SHIV (designated SHIV-33A), containing the env gene of HIV-1-SF33, was obtained by one passage in a juvenile rhesus macaque. SHIV-33A produced simian AIDS in juvenile macaques when given by either the intravenous or mucosal membrane (oral and vaginal mucosa) routes. In addition, newborn macaques (2 days of age) also developed simian AIDS after intravenous inoculation. Sequence analysis of the env gene of SHIV-33A revealed about 15 amino acid changes, relative to the input SHIV-33 clone. All animals showing immunodeficiency disease also exhibited a rapid and sustained decline in CD4 T-cells in both peripheral blood and lymph nodes. Future Directions Intragenic SHIV-33 recombinants and point mutants, involving the HIV-1SF33 env gene, will be constructed to determine which sequence change(s) accounts for viral adaptation and pathogenesis. Studies will also be performed to determine the mechanism of rapid depletion of CD4 T-cells in the acute stage of infection with the pathogenic SHIV-33A strain; this depletion may be due to either direct cytopathic effects of virus or to indirect mechanisms involving alterations in cytokines in the host. KEYWORDS SHIV, vaccine